CT scanners of the third-generation type include an X-ray source and X-ray detector system secured respectively on diametrically opposite sides of an annular disk. The latter is rotatably mounted within a gantry support so that during a scan the disk rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system typically includes an array of detectors disposed along an arc of a circle having a center of curvature at a focal point from which radiation emanates from the X-ray source. The X-rays that are detected by a single detector at a measuring instant during a scan are partially attenuated by the mass of all objects in their path. The detectors sense this attenuation and generate a single intensity measurement as a function of the attenuation, and thus the density, of the mass in the x-ray path.
For accurate image reconstruction from x-ray density data, the positions of the rays, and thus of the detectors, must be precisely known. Further, since dense matter tends to scatter X-rays, it is important that any radiation that does not traverse a straight line from the source to each detector be excluded from the measurements by each such detector. To remove this scattered radiation, a series of very thin x-ray opaque anti-scatter plates is typically inserted between the detectors and the object being scanned, with the individual plates aligned so as to collimate the x-rays from the radiation source by allowing to pass to the detectors substantially only those x-rays traversing a straight, radial line between the source and each detector.
Unfortunately, the need for the anti-scatter plates creates additional difficulties because if they cast an X-ray "shadow" on a detector, they will interfere with its measurements, unless all of the anti-scatter plates shadow all of the detectors uniformly. The output of each shadowed detector will be not only reduced, but also modulated by any detectable relative movement of the source, anti-scatter plates and/or detectors.
The difficulty of meeting these requirements is evident. In order to provide the resolution expected of modem X-ray tomographic scanners, hundreds of detectors are required, with several detectors located within a single degree of the arc of the x-ray beam. This makes the width of a typical detector on the order of a millimeter. The width of a typical anti-scatter plate is about ten percent of the width of a detector. The spaces between adjacent detectors are scarcely larger than that. Thus, extremely accurate detector and anti-scatter plate location and alignment is required. To further compound the problem, the whole assembly is usually rotated around the scanned object at a rate of about 60 to 120 rpm, generating substantial varying forces and requiring rugged mounting techniques. With the introduction of two-dimensional detector arrays, tolerance stacking occurs in two dimensions instead of only one, and it is even more critical to provide highly accurate positioning of the detectors and the anti-scatter plates.
A difficulty with high-resolution detector subsystems is obtaining and maintaining the relatively tight alignment requirements of the detectors and anti-scatter plates with the x-ray beams from the radiation source. Tolerances are further strained by any temperature and vibrational changes in the relative alignment of the subsystems.
Attempts have been made to facilitate the accurate location and alignment of detectors and anti-scatter plates in tomography systems. For example, U.S. Pat. No. 5,487,098 to Dobbs et al., assigned to the assignee of the present invention, discloses preassembled modules for the detector and anti-scatter plate arrays. The detector and anti-scatter plate modules are each attached to a support structure or spine which is then attached to a rotating gantry of the tomography system. Each detector module must be aligned with a corresponding anti-scatter plate module, and each pair of modules must be aligned relative to the focal spot, in order to maximize receipt of radiation.
U.S. Pat. No. 4,338,521 to Shaw et al. discloses a modular detector array that includes two detachably assembled portions, one containing the detectors and the other containing the anti-scatter plates. The two portions of the array must be assembled together in order to establish their mutual alignment. The assembled module must then be aligned with the radiation source and then fixedly mounted to the tomography apparatus.
U.S. Pat. No. 4,429,227 to DiBianca et al. discloses a modular x-ray detector which comprises a pair of diodes in a diode support frame, a pair of scintillator bars, and a collimator plate with extensions for engaging with a corresponding extension of the diode support frame. The collimator plate includes a pocket on each side for accommodating a scintillator bar. Each collimator plate/diode pair is independently mounted in slots in a pair of ceramic sections which are mounted to respective end members of an arcuate housing.
The prior art does not teach the use of a fully integrated detector/anti-scatter plate assembly in which the detectors and anti-scatter plates are intrinsically aligned with one another. It would therefore be advantageous to provide an x-ray detector and anti-scatter plate assembly which is fully integrated so that all of the diodes, scintillator crystals and anti-scatter plates are self-aligned as they are assembled together.